1. Field of the Invention
The present invention relates to an antenna system suitable for a communication system using a non-geostationary satellite such as a low orbit satellite or the like.
2. Description of the Prior Art
FIG. 1 and FIG. 2 show the antenna system used for the communication with a conventional non-geostationary satellite. Conventionally, with this kind of antenna system, as shown in FIG. 1, a parabolic antenna 1 is attached to a support 103 having at both ends an elevation angle adjusting mechanism 101 adjustable in the angle Y (angle of elevation) from the horizontal direction and an azimuth angle adjusting mechanism 102 adjustable in the angle X (azimuth angle) in the horizontal direction, via the elevation angle adjusting mechanism 101.
Therefore, with the conventional antenna system, the elevation angle adjusting mechanism 101 and the azimuth angle adjusting mechanism 102 of the antenna are provided for each antenna, and the direction of the antenna is adjusted by adjusting the two adjusting mechanisms 101 and 102.
Therefore, at the time of communication with the non-geostationary satellite, when the direction of the communication target as seen from a reception point (where the antenna is set up) changes with the lapse of time, it is necessary to have the same number of antenna systems as shown in FIG. 1 as that of the communication targets, in order to communicate simultaneously with a plurality of communication targets which are in the different direction as seen from the reception point.
However, setting of a plurality of antennas including the direction adjusting mechanism has such problems that not only it simply takes up a lot of space, but also there is a case where antennas become an obstacle to the communication with each other, depending upon the positional relationship of the antennas and the direction of the communication target.
Therefore, there has been proposed an antenna system 1A having a construction shown in FIG. 2 in which two antenna systems are placed on the same turntable 105, and the turntable 105 is rotated so that the antennas 1a and 1b do not become an obstacle to the communication with each other.
However, with the construction of the antenna system 1A as shown in FIG. 2, five movable adjusting sections are required for adjusting the direction of the antennas 1a and 1b. Hence, there is a problem that the mechanism becomes complicated and the direction control of the antenna (particularly, the control of the azimuth angle X) becomes complicated.
Moreover, with the satellite communication using a non-geostationary satellite, since the position of the target satellite to be communicated with as seen from the reception point on the ground changes with the lapse of time, it is necessary to follow the target satellite and direct the antenna accurately to the satellite for continuing the communication (for example, see Japanese Patent Application Laid-open Hei 9 No. 321523).
When the satellite used for the communication is to be replaced, an operation for searching and acquiring a new satellite is necessary. Even if the orbit information of the satellite is known and estimate of the position of the satellite is possible by means of calculation, there is a subtle difference between the calculated value and the actual value, hence follow up and acquisition of a satellite cannot be done so easily by means of an antenna having a high directivity used in a ground station of the satellite communication.
Therefore, an auxiliary antenna for acquiring and following a satellite having a Low directivity compared to an antenna for communication (hereinafter referred to as a "pilot antenna") is provided in addition to the antenna for communication, and the actual position of the satellite is acquired by using the pilot antenna in advance, at the time of adjusting the direction of the antenna for communication.
However, even if the pilot antenna is used, the direction control mechanism of each antenna is independent and the set up position of the two antennas are different. Hence, when it is tried to direct the antenna for communication to the same direction as that of the pilot antenna which has acquired a satellite, directional control of the antenna becomes complicated.